Electrophotographic reversal development process for enhancing the quality of the developed image

ABSTRACT

An electrophotographic process occurring subsequent to the liquid reversal development of a photoconductive insulating layer comprising majority and minority charge carriers with a toner having a polarity opposite to that of the minority charge carriers characterized by charging evenly the reversely developed surface of the photoconductive insulating layer with corona ions of the same polarity as that of the minority charge carriers, and subsequently squeezing the residual liquid adhering to the insulating layer from the liquid reversal development step by means of squeeze rollers.

United States Patent 1191 Sato et a1. 5] Apr. 16, 1974 ELECTROPHOTOGRAPHIC REVERSAL 3,560,203 2/1971 Honjo etal. 117/37 LE DEVELOPMENT PROCESS FOR 3,655,419 4/1972 Tamai et a1 117/37 LE 3,444,369 5/1969 Malinarc 96/l.4 ENHANCING THE QUALITY OF THE 3,650,622 3/1972 Morse 117/37 LE DEVELQPED IMAGE 3,663,219 5/1972 Takahashi 96/14 75 Inventors: Masamichi s w; Yasuo Tamai; 3,383,209 5/1968 Cassiers et a1. 117/37 LE 05am Fukushima; fi 3,425,829 2/1969 Cassrers et a1. 117/37 LE Matsumoto; Kazuo Horikawa, all of Asaka, Japa Primary Examiner-William D. Martin Assistant Examiner-M. Sofocleous [73] Asslgnee: Full Attorney, Agent, or FirmMartin: .1. T.; Gerald .1. Per- Kanagawa Japan guson, Jr.; Joseph J. Baker [22] Filed: Dec. 10, 1971 [21] Appl. No.: 205,996 [57] ABSTRACT An electrophotographic process occurring subsequent [30] Foreign Application Priority Data to the liquid reversal development of a photoconduc- Dec. 10, 1970 Japan 45-109877 five insulating layer comprising majority and minority charge carriers with a toner having a polarity opposite 52 us. Cl 117/37 LE, 96/1 LY that of the minority charge carriers Characterized [51] Int. Cl G03g 13/10 charging evenly the reversely developed Surface of [58] Field of Search 117/37 LE; 96/] LY; the photoconductive insulating layer with corona ions 355 n 1 18/637 of the same polarity as that of the minority charge carriers, and subsequently squeezing the residual liquid [56] References Cited adhering to the insulating layer from the liquid rever- UNITED STATES PATENTS sal development step by means of squeeze rollers.

3,722,994 3/1973 Tanaka et a1. 1 17/37 LE 4 Claims, 3 Drawing Figures ELECTROPHOTOGRAPHIC REVERSAL DEVELOPMENT PROCESS FOR ENHANCING THE QUALITY OF THE DEVELOPED IMAGE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an electrophotographic squeezing process aimed at stabilizing an image developed with liquid developer.

2. Description of the Prior Art Electrophotographic liquid developers are usually involved in processes as are described below.

An electrophotographic material with an electrostatic latent image formed thereon is first dipped in a tonerless insulating liquid and wetted on both sides. This practice is called pre-bathing. The purpose of prebathing is to form a thin film to prevent adhesion of the toner suspended in the liquid developer to the electrophotographic material. As the pre-bathing liquid, kerosene, cyclohexene, benzene, pentane, decalin, and many other non-polar hydrocarbons and halogenated hydrocarbons have been used.

The electrophotographic material that has undergone the pre-bathing process is then dipped in the developer. The developer is composed of an insulating liquid (called a carrier liquid) with fine chargeable particles (toner) suspended therein.

Usually a charge controlling agent intended to control the charging of the toner or a dispersion stabilizer intended to stabilize the dispersion is also present in the carrier liquid, either in suspension or dissolved. As charge controlling agents or dispersion stabilizers, resins, varnishes or non-drying oils which are soluble in the carrier liquid, or resins, varnishes or the like which are not soluble in the carrier liquid have been used.

The carrier liquid is usually the same liquid used for prebathing. When the charge of the toner has the same sign as the electrostatic latent image, deposition of the toner onto noncharged or slightly charged regions of the latent image occurs, which is called reversal or repulsion development. In contrast, when the charge of the toner is opposite that of the latent image, the toner is deposited onto the charged region of the latent image, which is called normal or attraction devel, opment. l

The toner particles, which are colorless or colored inorganic or organic pigments, are suspended in the carrier liquid, either by themselves or in such a form that resin or varnish is absorbed on the surface thereof.

Development is completed when the toner is sufficiently deposited on the latent image surface, followed by a rinsing or squeezing process.

Rinsing is accomplished either by dipping the developed electrophotographic material into a rinse or by spraying a rinse over the developed electrophotographic material. A quick-drying insulating liquid with low solubility for the toner is utilized as the rinse. Of the many of such rinsing liquids available, difluorotetrachlorethane, trifluorotrichloroethane and Isoper E (commercial name used by Esso-Standard Oil Co., Ltd.) are recommended. The purpose of rinsing is to remove the developer deposited on the surface of the electrophotographic material and to enhance the adhesiveness of the toner-developed image. The reason the toner-developed image is increased in adhesiveness is that the resin or varnish absorbed on the toner particles is fixed without being dissolved.

The electrophotographic material that has been rinsed is passed through squeeze rollers, whereby its surface is cleared of the developer deposited thereof and is almost dried. If the electrophotographic material is passed through the squeeze rollers immediately after being developed without being rinsed, the tonerdeveloped image often is transferred to the squeeze rollers or is crushed and oozes out. Even if rinsing is done, a too quick rinse, or an undesirable choice of the rinse can also give rise to the aforementioned troubles. Such trouble is liable to particularly occur when an electrophotographic material that has been reversal developed is squeezed. Normal development is hardly ever accompanied by such a problem.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sectional view of an electrophotographic material with toner particles deposited on the surface thereof.

FIG. 2 is a sectional view equivalent to FIG. 1 illustrating the principle of the process of the invention.

FIG. 3 is a sectional view of apparatus useful for the practice of the present invention.

FIG. 1 illustrates a model intended to indicate the difference in the resistance of a toner image to squeezing between normal development and reversal development.

Electrophotographic material 10 is composed of a photoconductive insulating layer 11 and a conductive base 12. A mixture of photoconductive powder and an insulating resin or an organic photoconductive compound is utilized to provide the photoconductive insulating layer. As the conductive base, a plastic film coated with a conductive polymer, or a metal-vacuum deposited or metal plated paper or a paper impregnated with conductive polymer may also be employed, as may a metalized paper.

In FIG. 1, the photoconductive insulating layer 11 is assumed to be an n-type semiconductor. A layer coated with a mixture of zinc oxide powder and insulating resin is such an n-type semiconductor. The latent image is formed by negative charge 13. The charge 14 is a positive counter charge, nearly equivalent to the negative charge 13, which is concentrated at the boundary between the photoconductive insulating layer 11 and the base 12. The negative charge 13 exists mostly on the surface of the photoconductive insulating layer 11.

If such a latent image is developed with a toner having a positive charge, the toner is attracted to the charged region of the latent image and is deposited thereon. This case is not of reversal development. Reference character 15 represents such a toner attracted by the charge 13 of the latent image and deposited the force under Coulombs law between the positive charge shifted to the boundary surface between the photoconductive insulating layer 11 and the base 12 as a result of the deposition of toner 16. The adhesive forces acting on toner 16 are very small. The Coulombic force is small because the distance between the toner l6 and the charge 13, or between toner 16 and the positive charge on the boundary surface thereunder, is long. In contrast, in the previously mentioned non-reversal development, the Coulombic attractive force between the toner l and the charge 13 is very large because the distance between the toner l5 and the charge 13 is very short.

SUMMARY OF THE INVENTION Based on the concept described above, we tried to increase the adhesiveness of the toner image to the latent image surface. Asa result, we have succeeded in enhancing the resistance of the toner image to the roller squeezing step to a marked extent.

FIG. 2 is a view equivalent to FIG. I intended to illustrate the principle of the present invention.

Device 20 is a known and available corona charging unit composed of a corona electrode 21 and a shield case 22. 23 is a DC high voltage powder source used to apply a positive high voltage to the corona electrode 21. Positive corona ions resulting from the corona discharge are deposited on the toner 16 shown in FIG. 1, changing the negative polarity of toner 16 to a positive polarity. Reference character 24 represents the toner l6 changed to have a positive charge. Simultaneous with the deposition of positive corona ions onto the negative polarity toner 16, a negative charge 25 equivalent thereto is imparted from the conductive layer 12 into the photoconductive insulating layer 11 (l 100 [1. thick), through which it arrives very close to toner 16.

For a negatively charged toner what is important here is that the insulating layer 11 be an n-type semiconductor and the electrons are free to move. That is, n-type photoconductive insulating layer on which a negative latent image is formed is developed with negatively charged toner and then the image bearing layer is exposed to positive corona ion. Even if negative corona ions are made to deposit on the toner 16 by applying negative high voltage to the corona no positive charge equivalent thereto is imparted into the insulating layer 1 1. Therefore, the Coulombic force is low due to long distance between the negative charge and the positive charge of the toner 16.

If the photoconductive insulating layer 11 is a p-type semiconductor such as selenium or polyvinylcarbazole, a positive latent image is formed thereon and developed with positively charged toner. In this case, a developer made by dispersing resins, pigments and preferably heavy metal salts of organic acid into carrier liquid, as described in US. Pat. No. 3,257,322 or British Pat. No. 1,065,795. And then negative high voltage is applied to the corona electrode 21 so that the developed layer is exposed to negative corona ion.

When the photoconductive insulating layer is n-type the layer must be positively charged after development and when p-type, the layer must be negatively charged.

FIG. 3 is a sectional view of equipment which can be used for this embodiment of the present invention.

Rollers 30 and 31 are a pair of feed rollers used to feed the developed electrophotographic material 10 held therebetween. The roller 30 is flanged at both ends to prevent the surface thereof from contacting the toner image, and the electrophotographic material is fed while held between the flanges of the roller 30 and the roller 31.

Nozzles 32 and 33 are used to spray the rinse liquid over the top and back surfaces of the electrophotographic material.

Rollers 34 and 35 are a pair of feed rollers similar to rollers 30 and 31. Roller 34 is flanged at both ends.

Reference characters 36 and 37 denote a corona wire and a shield plate respectively.

Rollers 38 and 39 are pair of squeeze rollers. At least one of rollers 38 and 39 is covered with an elastic soft material (rubber, sponge, etc.). Rollers 30, 31, 34, 35 and 38 can be made of a hard material, such as metal or plastic.

The electrophotographic material is washed with the rinse between rollers 30 and 31 and rollers 34 and 35. Subsequently to rollers 34 and 35 it has the toner image surface thereof exposed to the corona discharge, and is thereafter squeezed to be dried.

In case the deposited toner layer is highly conductive, the process of the present invention is not effective. Generally, the present invention can be practiced as long as the toner layer deposited on the photoconductive layer shows a surface resistance of l0Q/square or higher.

Of developers having good charging characteristics, it is only those chiefly composed of carbon with a low resin content that provide a toner layer that shows a surface resistance lower than 10 Q/square. Almost all others composed of organic or inorganic pigments satisfy the requirement mentioned above. One skilled in the art can thus easily determine the acceptability or non-acceptability of various developers by a simple resistivity measurement. Carbon Black type developers, of course, can easily satisfy the requirement mentioned above subject to the choice and content of resinous material.

Example I A sheet of art paper was coated with a mixture obtained by mixing weight parts of photoconductive zinc oxide powder with 20 weight parts of styrenated alkyd resin (Styresol 4400 trade name of Nippon Reichhold Co.) so that the dried thickness was approximately 7 microns. The art paper used had earlier been coated with a polymeric quaternary ammonium salt (a conductive polymer known as Calgon Conductive Polymer 261 manufactured by Calgon, U.S.A.) to provide a dried coverage of 1.5 g/m This electrophotographic material was charged to a surface potential of -180 V by a negative corona discharge in the dark and then exposed to imaging light, whereby a negative latent image was formed thereon. Subsequently, it was dipped into a developer containing a toner having a negative charge and developed for 1 minute.

The developer was prepared by the following procedure:

A liquid dispersion composed of:

Nitrocellulose 4 weight parts -Continued Carbon Black Acetone 1 weight part 50 weight parts was dried into a fine powder. Then, weight parts of this fine powder was dispersed in a ball mill for 50 hours with:

Safflower oil-modified alkyd resin 200 weight parts (Super Beckosol 1-537, trademark material manufactured by Nippon Reichhold Co.)

lsoper H I00 weight parts (an Esso-Standard Oil Co. isoparaffinic solvent) Rollers 31, 35 and 38 were l6 mm in diameter and were of stainless steel. Rollers 30 and 34 were 16 mm in diameter, the flanges thereof being 18 mm in diameter, and were of stainless steel. Roller 39 was 16 mm in outer diameter and was covered with 4 mm thick rubber. A stainless steel corona electrode of 0.1 mm diameter was used.

.The corona electrode was impressed with a positive voltage of 8 kV with the corona electrode approximately 10 mm from the electrophotographic material. The electrophotographic material was fed at a rate of mm per second, but the toner image withstood squeezing by rollers 38 and 39.

In the case where no corona discharge or a negative voltage was applied to the corona wires, the toner image was destroyed upon squeezing.

Example II Carbon Black Phthalocyanine Blue 0.4 weight parts 0.05 weight parts Ethyl Cellulose 1 weight part Ethyl Acetate 20 weight parts Toluene 19 weight parts In this developer, the pigments are covered by the ethyl cellulose and its polarity as toner was positive.

The latent image bearing paper was developed in such developer for 20 sec. After the development, the electrophotographic paper is rinsed in lsoper G (trade name of Esso Standard Oil Co.; isoparaffinic solvent whose evaporation speed is much higher than that of lsoper H) and exposed to negative corona ion in wet state. The corona discharge electrode of molybdenum wire of 30 p. d) was kept at 15 mm apart from the paper, and applied a voltage of minus 7 KV. And then lsoper G sticked on the paper was removed by the squeeze rollers. An excellent negative image to original image was obtained.

In the present invention, exposure to corona ion after the development may be processed after rinse as described above or may be done immediately after the development eliminating the rinsing step. But the rinse is advantageous in making the paper dry fast and preventing fog on the image.

Numerous modifications of the invention will become apparent to one of ordinary skill in the art upon reading the foregoing disclosure. During such a reading it will be evident that this invention provides a unique electrophotographic post-development process for accomplishing the objects and advantages herein stated.

What is claimed is:

1. In an electrophotographic process including the liquid reversal development of a photoconductive insulating layer disposed on a conductive substrate, said insulating layer comprising majority and minority charge carriers with a toner having a polarity opposite to that of said minority charge carriers and subsequently squeezing the residual liquid adhering to said insulating layer from the liquid reversal development step by means of squeeze rollers, the improvement comprising, prior to said squeezing step, charging evenly the reversely developed surface of the photoconductive insulating layer with corona ions of the same polarity as that of the minority charge carriers to thereby reverse the polarity of said toner and enhance the electrostatic attraction between the toner and the insulating layer and thus improve the resistance of the toner image to the said squeezing step.

2. An electrophotographic process as claimed in claim 1 including rinsing the reversely developed insulating layer with an isoparaffinic solvent prior to said charging step.

3. A process as in claim 1 where said photoconduc tive insulating layer is n-type, the polarity of said toner is negative, and the polarity of said corona ions is positive. 

2. An electrophotographic process as claimed in claim 1 including rinsing the reversely developed insulating layer with an isoparaffinic solvent prior to said charging step.
 3. A process as in claim 1 where said photoconductive insulating layer is n-type, the polarity of said toner is negative, and the polarity of said corona ions is positive.
 4. A process as in claim 1 where said photoconductive insulating layer is p-type, the polarity of said toner is positive, and the polarity of said corona ions is negative. 